Ultraviolet-curable adhesive composition for bonding opaque substrates

ABSTRACT

An ultraviolet-curable adhesive composition comprising a photopolymerization initiator whose molar absorption coefficient at a wavelength of 360-450 nm is at least 400 and an ultraviolet-curable compound. The adhesive composition of this invention can bond substrates which show transmittances of 0.01-20% to all of energy beams of a wavelength of 280-380 nm, to one another such as base boards for an optical disk.

This application is Continuation of U.S. Ser. No. 09/170,320 filed Oct.13, 1998, abandoned which is a Divisional of 08/765,016 filed Dec. 19,1996, which is a 371 of PCT/JP96/01134 filed Apr. 25, 1996.

TECHNICAL FIELD

This invention relates to an ultraviolet-curable adhesive, moreparticularly to an adhesive capable of bonding substrates which showtransmittances of 0.01 to 20% to all of energy beams having wavelengthsof not less than 280 nm but not more than 380 nm, to one another.

BACKGROUND ART

When opaque substrates are bonded to one another, it is difficult to useusual photocuring adhesives because the substrates do not sufficientlytransmit lights. In this case, therefore, heat-bonding is generallyadopted. In the heat-bonding, a resin of epoxy type, melamine type,alkyd type, urethane type or acrylate type and a heat-curing agent areused in many cases and the use of a hot-melt type resin or the like isalso known.

However, in the heat-bonding, there have been such problems that thesubstrates are warped or distorted owing to heat, and hence, the kind ofsubstrate to which the heat-bonding can be applied has been limited.Moreover, the hot-melt type resin is inferior in thermal stability andweather resistance, and has been difficult to use under high temperaturecircumstances. In addition, the heat-bonding has required a curing timeof several tens of minutes and hence has had a problem that it isinferior in productivity.

Therefore, there has been desired a development of an adhesive and abonding method which are free from such problems as warpage anddistortion of substrate, productivity and the like. In particular, inorder to bond optical discs which will be required to have a higherrecording density in future, there must be used an adhesive which causesneither warpage nor distortion of disc and is endowed with excellentcharacteristics.

DISCLOSURE OF THE INVENTION

The present inventors have made extensive research for solving the aboveproblems; have consequently found that contrary to the prior commonknowledge that opaque substrates cannot be bonded with a photocurableresin, opaque substrates which even slightly transmit light can bebonded with a photocurable resin; and have made this invention.

This invention is an ultraviolet-curable adhesive composition comprisingat least one photopolymerization initiator whose molar absorptioncoefficient at a wavelength of 360 nm or more, particularly 360-400 nmis at least 200 and an ultraviolet-curable compound.

This invention is also an ultraviolet-curable adhesive compositionconsisting of at least one photopolymerization initiator whose molarabsorption coefficient at a wavelength of 360-450 nm is at least 400 andan ultraviolet-curable compound.

This invention is further a method for bonding substrates whichcomprises coating the above-mentioned ultraviolet-curable adhesivecomposition on a substrate which shows transmittances of 0.01 to 20% toall of energy beams having wavelengths of 280-380 nm, thereafter firmlycontacting the coating surface of this substrate with another substrateand then irradiating them with ultraviolet rays.

BEST MODE FOR CARRYING OUT THE INVENTION

The ultraviolet-curable adhesive composition of this invention comprisesat least one photopolymerization initiator whose molar absorptioncoefficient at a wave-length of 360 nm or more, particularly 360-400 nm,is at least 200.

In another aspect, the ultraviolet-curable adhesive composition of thisinvention comprises at least one photopolymerization initiator whosemolar absorption coefficient at a wavelength of 360-450 nm is at least400.

Here, that the molar absorption coefficient in a range of theabove-mentioned absorption wavelengths is at least 200 or at least 400means that in this wavelength region, there is at least a wavelengthportion at which molar absorption coefficient is at least 200 or atleast 400, respectively, and it is not necessary that the molarabsorption coefficient is at least 200 or at least 400 over the wholewavelength range. The content of the above photopolymerization initiatorin this invention is usually 0.01-20% by weight, preferably 0.1-20% byweight, more preferably 0.5-20% by weight, and further preferably0.5-10% by weight, approximately, in the ultravioletcurable adhesivecomposition. Theses photopolymerization initiators may be used alone orin admixture of two or more in any proportion. As far as theabove-mentioned conditions are satisfied, other polymerizationinitiators may be co-used.

As the photopolymerization initiator whose molar absorption coefficientat a wavelength of 360 nm or more is at least 200, there can bementioned, for example, camphorquinone,2-methyl-1-(4-methylthiophenyl)-2-(4-morphonyl)-1-propanone,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone-1,2-chlorothioxanthone, 2,4-dimethylthioxanthone,2,4-diisopropylthioxanthone, isopropylthioxanthone,2,4,6-trimethylbenzoyldiphenylphosphine oxide,bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide,(1-6-η-cumene)(η-cyclopentadienyl)iron (1+) hexafluorophosphate (1-) andthe like.

In particular, as the photopolymerization initiator whose molarabsorption coefficient at a wavelength of 360-400 nm is at least 200,there can be mentioned, for example,2-methyl-1-(4-methylthiophenyl)-2-(4-morphonyl)-1-propanone,2-benzyl-2-dimethylarnino-1-(4i-morpholinophenyl)butanone-1,2-chlorothioxanthone, 2,4-dimethylthioxanthone,2,4-diisopropylthioxanthone, isopropylthioxanthone,2,4,6-trimethylbenzoyldiphenylphosphine oxide,bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide,(1-6-η-cumene)(η-cyclopentadienyl)iron (1+) hexafluorophosphate (1−) andthe like.

The photopolymerization initiator used in this invention may be one ofthose whose molar absorption coefficient at a wavelength of 360-450 nmis at least 400. Among them, preferable initiators are compounds whosemolar absorption coefficient at a wavelength of 360-450 nm is at least500, for example,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,2-chlorothioxanthone, 2,4-diethylthioxanthone,2,4-diisopropylthioxanthone, isopropylthioxanthone,2,4,6-trimethylbenzoyldiphenylphosphine oxide andbis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide. Morepreferable initiators are compounds whose molar absorption coefficientat a wavelength of 360-450 nm is at least 1,000, for example,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone-1,2-chlorothioxanthone, 2,4-diethylthioxanthone,2,4-diisopropylthioxanthone, isopropylthioxanthone andbis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.

In this invention, such photopolymerization initiating assistants asamines and the like can be co-used. Such photopolymerization initiatingassistants as amines and the like include, for example,2-dimethylaminoethylbenzoate, dimethylaminoacetophenone, ethylp-dimethylaminobenzoate, isoamyl p-dimethylaminobenzoate and the like.The amount of the photopolymerization initiating assistant used isusually preferably 0-15% by weight, more preferably 0-10% by weight,approximately, in the composition.

The ultraviolet-curable adhesive composition of this inventioncomprises, in addition to the above-mentioned photopolymerizationinitiator, an ultraviolet-curable compound. The ultraviolet-curablecompound includes, for example, monomers having one or more(meth)acryloyl groups in the molecule. When a higher bonding strengthand higher durability (deterioration resistance) is required, anoligomer can be co-used.

The monomers having one or more (meth)acryloyl groups in the moleculeinclude monofunctional acrylic monomers having one (meth)acryloyl groupin the molecule and polyfunctional acrylic monomers having two or more(meth)acryloyl groups in the molecule.

The monofunctional acrylic monomer includes, for example, acrylicmonomers having a ring structure such as aliphatic ring, aromatic ring,heterocyclic ring or the like, aliphatic acrylates having a hydroxylgroup and the like.

The acrylic monomer having a ring structure such as aliphatic ring,aromatic ring, heterocyclic ring or the like includes, for example,tricyclodecane (meth)acrylate, dicyclopentenyl (meth)acrylate, isobornyl(meth)acrylate, adamantyl (meth)acrylate, phenyl (meth)acrylate, benzyl(meth)acrylate, tetrahydrofurfuryl (meth)acrylate, morpholine acrylate,phenylglycidyl (meth)acrylate and the like. Alkylene oxide-modificationproducts of them can also be used. In particular, those in which thealkylene oxide has 2-3 carbon atoms, are preferred, and include, forexample, dicyclopentenyloxyethyl (meth)acrylate, phenyloxyethyl(meth)acrylate and the like.

The aliphatic acrylate having a hydroxyl group is preferably, forexample, an acrylate in which a hydroxyl group is bonded to an aliphaticgroup having 2-9 carbon atoms, more preferably an acrylate in which ahydroxyl group is bonded to an aliphatic group having 2-4 carbon atoms.This aliphatic acrylate may have bonded thereto a substituent such as aphenoxy group. The aliphatic acrylate having a hydroxyl group includes,for example, 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl(meth)acrylate, 4-hydroxybutyl (meth)acrylate/ 2-hydroxy-3-phenoxypropyl(meth)acrylate, and the like.

Among these monofunctional acrylic monomers, particularly preferable forkeeping viscosity, resistance to moist heat and adhesiveness, are, forexample, phenyloxyethyl (meth)acrylate, tricyclodecane (meth)acrylate,isobornyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, morpholineacrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl(meth)acrylate and the like.

The polyfunctional acrylic monomers are classified into difunctionalacrylic monomers and trifunctional or more functional acrylic monomers.

The difunctional acrylic monomers include, for example, acrylates ofaliphatic diols having 4-9 carbon atoms, alkylene oxide type acrylicmonomers and acrylic monomers having a ring structure.

The acrylates of aliphatic diols having 4-9 carbon atoms include, forexample, neopentylglycol di(meth)acrylate, 1,6-hexanedioldi(meth)acrylate and the like. This acrylate of the aliphatic diol maybe modified with an aliphatic ester or an alkylene oxide. The aliphaticester-modified acrylates include, for example, neopentylglycolhydroxypivalate di(meth)acrylate, caprolactone-modified neopentylglycolhydroxypivalate di(meth)acrylate and the like. The alkyleneoxide-modified acrylate compounds include, for example, diethyleneoxide-modified neopentylglycol di(meth)acrylate, dipropyleneoxide-modified neopentylglycol di(meth)acrylate, diethyleneoxide-modified 1,6-hexanediol di(meth)acrylate, dipropyleneoxide-modified 1,6-hexanediol di(meth)acrylate and the like.

The alkylene oxide type acrylic monomers include, for example,neopentylglycol-modified trimethylolpropane din(meth)acrylate,polyethylene glycol di(meth)acrylate, polypropylene glycoldi(meth)acrylate and the like. The acrylic monomers having a ringstructure include, for example, tricyclodecanedimethyloldi(meth)acrylate, dicyclopentanyl di(meth)acrylate and the like.

The trifunctional and more functional acrylic monomers include, forexample, trimethylolpropane tri(meth)acrylate, pentaerythritoltri(meth)acrylate, C₂₋₅aliphatic modified dipentaerythritolpenta(meth)acrylate, C₂₋₅aliphatic modified dipentaerythritoltetra(meth)acrylate, dipentaerythritol penta(meth)acrylate,dipentaerythritol hexa(meth)acrylate, caprolactone-modifieddipentaerythritol hexa(meth)acrylate, pentaerythritoltetra(meth)acrylate, tris[(meth)acryloxyethyl] isocyanurate,caprolactone-modified tris[(meth)acryloxyethyl] isocyanurate,ditriimiethylolpropane tetra(meth)acrylate and the like.

Among these polyfunctional acrylic monomers, particularly preferabledifunctional acrylic monomers for keeping viscosity, resistance to moistheat and adhesiveness include, for example, acrylates of aliphatic diolshaving 4-9 carbon atoms such as neopentylglycol di(meth)acrylate,1,6-hexanediol di(meth)acrylate and the like;

and aliphatic ester-modified aliphatic diol acrylates such asneopentylglycol hydroxypivalate di(meth)acrylate, caprolactone-modifiedneopentylglycol hydroxypivalate di(meth)acrylate and the like, andparticularly preferable trifunctional or more functional acrylicmonomers for keeping viscosity, resistance to moist heat andadhesiveness include, for example, dipentaerythritolpenta(meth)acrylate, dipentaerythritol hexa(meth)acrylate,tris-[(meth)acryloxyethyl] isocyanurate and caprolactone-modifiedtris[(meth)acryloxyethyl] isocyanurate.

The amount of these monomers used is usually preferably about 5 to 90%by weight. These monomers may be used alone or in admixture of two ormore in any proportion. In view of viscosity, the use of themonofunctional or difunctional acrylic monomer is preferred, and thetrifunctional or more functional acrylic monomer is used as necessary.

When the adhesive composition of this invention is required to have ahigher bonding strength and a durability (deterioration resistance) asmentioned above, an oligomer can be co-used. The oligomer which can beused in this invention is preferably one which can be dissolved in themonomer, and also preferably one having at least two (meth)acryloylgroups in the molecule. Such oligomers include, for example, epoxy(meth)acrylate, polyester (meth)acrylate and urethane (meth)acrylate.

The proportions of the monomer and the oligomer used are such that theproportion of the monomer is preferably 20-100 w/w %, more preferably20-95 w/w % and further preferably 50-95 w/w %, and the proportion ofthe oligomer is preferably 0-80 w/w %, more preferably 5-80 w/w % andfurther preferably 5-50 w/w %. If necessary, a high molecular weightpolymer, an additive and the like can also be used.

The epoxy (meth)acrylate is obtained by the reaction of an epoxy resinwith (meth)acrylic acid. The epoxy resin includes, for example,bisphenol type epoxy resins such as bisphenol A type epoxy resin,bisphenol F type epoxy resin and the like; novolak type epoxy resins.The bisphenol A type epoxy resin includes, for example, Epikote 828(trade name), Epikote 1001 (trade name) and Epikote 1004 (trade name)manufactured by Yuka Shell Epoxy Co., Ltd., and the like, the bisphenolF type epoxy resin includes, for example, Epikote 4001P (trade name),Epikote 4002P (trade name) and Epikote 4003P (trade name) manufacturedby Yuka Shell Epoxy Co., Ltd., and the like. The novolak type epoxyresin includes, for example, Epikote 152 (trade name) and Epikote 154(trade name) manufactured by Yuka Shell Epoxy Co., Ltd., and the like.

The polyester (meth)acrylate is obtained by the reaction of apolyesterpolyol with (meth)acrylic acid. The polyesterpolyol is obtainedby the reaction of a polyhydric alcohol with a polybasic acid. Thepolyhydric alcohol includes, for example, neopentyl glycol, ethyleneglycol, propylene glycol, 1,6-hexanediol, trimethylolpropane,pentaerythritol, tricyclodecanedimethylol, bis(hydroxymethyl)cyclohexaneand the like. The polybasic acid includes, for example, succinic acid,phthalic acid, hexahydrophthalic anhydride, terephthalic acid, adipicacid, azelaic acid, tetrahydrophthalic anhydride and the like.

The urethane (meth)acrylate includes those obtained by the reaction ofthree compounds of a polyol, an organic polyisocyanate and ahydroxy(meth)acrylate and by the reaction of two compounds of an organicpolyisocyanate and a hydroxy(meth)acrylate without using the polyol. Thepolyol includes polyether polyols such as polypropylene glycol,polytetramethylene glycol and the like; polyester polyols obtained bythe reaction of the above-mentioned polyhydric alcohols with theabovementioned polybasic acids; caprolactone polyols obtained by thereaction of the above-mentioned polyhydric alcohol with theabove-mentioned polybasic acid and ε-caprolactone; and polycarbonatepolyols (for example, polycarbonate polyol obtained by 1,6-hexanediolwith diphenyl carbonate, and the like); etc. The organic polyisocyanateincludes, for example, isophorone diisocyanate, hexamethylenediisocyanate, tolylene diisocyanate, xylene diisocyanate,diphenylmethane-4,4′-diisocyanate, dicyclopentanyl diisocyanate and thelike. Those obtained by the reaction of the three compounds and thoseobtained by the reaction of the two compounds can be used each alone orin combination of the two.

Among these oligomers, particularly preferable for keeping theviscosity, resistance to moist heat and adhesiveness are, for example,epoxy (meth)acrylates and urethan (meth)acrylates.

These oligomers may be used alone or in admixture of two or more in anyproportion. The amount of the oligomer used is usually, preferably about0-70% by weight.

The adhesive composition of this invention can have added thereto, ifnecessary, for example, a polyester resin, a polycarbonate resin, apolyacrylic resin, a polyurethane resin, a polyvinyl resin or the likeas the high molecular weight polymer, and moreover, if necessary, suchadditive as a silane coupling agent, a polymerization inhibitor, aleveling agent, a surface lubricating agent, a defoaming agent, a lightstabilizer, an antioxidant, an antistatic agent, a filler and the likecan also be co-used.

The silane coupling agent includes those of an alkyl type, an aminetype, a (meth)acrylate type, an isocyanate type, an epoxy type, a thioltype and the like. The amount of this silane coupling agent used isabout 0-10% by weight in the composition. The polymerization inhibitorincludes methoquinone, methylhydroquinone and the like. The amount ofthe polymerization inhibitor used is about 0-1% by weight in thecomposition. The leveling agent, surface lubricating agent and defoamingagent include those of an organic polymer type, a silicone type, afluorine type and the like. The antioxidant includes those of a hinderedamine type, a hindered phenol type, a high molecular weight phenol typeand the like. The amount of each of the above leveling agent, surfacelubricating agent, defoaming agent and antioxidant used is about 0-5% byweight in the composition. The antistatic agent includes those of aquaternary ammonium type, a polyether type, a conductive powder and thelike. The amount of the antistatic agent used is about 0-30% by weightin the composition. The filler includes silica gel, titanium oxide,alumina, conductive powder and the like. The amount of the filler usedis about 0-70% by weight in the composition. The amounts of theseadditives used are appropriately determined in the above mentionedranges depending upon the purposes.

The adhesive composition of this invention can be obtained by mixing theabove-mentioned components at a temperature from room temperature to 80°C. to dissolve or disperse them. The adhesive composition of thisinvention can usually be used in the form of a mixture (solution,dispersion or the like) of the above-mentioned components as it is;however, if necessary (for example, for adjusting the viscosity of thecomposition or assuring the smoothness after coating), the adhesivecomposition may be used in solution or dispersion in an organic solvent.The organic solvent which can be used includes, for example, toluene,xylene, methyl ethyl ketone, isopropanol, diethylene glycol monoethylether acetate, diethylene glycol monobutyl ether, diethylene glycolmonobutyl ether acetate, diethylene glycol monodiethyl ether and thelike. When the organic solvent is used, the amount used is appropriatelydetermined depending upon the purpose.

The cured product of the adhesive composition of this invention can beobtained by irradiation with a light such as an ultraviolet ray, avisible laser or the like in the conventional manner. The curing of theadhesive composition of this invention by irradiation with a light raysuch as an ultraviolet ray or the like is specifically conducted byirradiation with an ultraviolet ray using a low pressure or highpressure mercury lamp, a metal halide lamp, a xenon lamp or the like. Inparticularly, the light source is preferably a lamp having a high energyintensity at 360-450 nm.

The adhesive composition of this invention is useful particularly as anadhesive for bonding opaque substrates which show transmittances of 0.01to 20% at all wavelengths in a region of not less than 280 nm but notmore than 380 nm, to each other. Of course, it is also applicable to thecase where one of the substrates is transparent and the other is anopaque substrate. A particularly suitable substrate is a base board foroptical disc.

The ultraviolet-curable adhesive composition of this invention can bondsubstrates to each other by coating the composition on an opaquesubstrate by use of a coating means such as a roll coater, a spincoater, a screen printing method or the like so that the thickness of adried coating film becomes 1-50 μm, firmly contacting another substratewith this coating surface, and subsequently irradiating them with anultraviolet ray from the upside of the opaque substrate to cure theadhesive composition. The curing of the adhesive composition of thisinvention can also be effected using a visible laser in place of theultraviolet ray. Also, the handling of the adhesive composition of thisinvention including preparation, storage, coating and the like ispreferably effected under the circumstances that light rays in thewavelength region ranging from visible to near-ultraviolet, particularlylight rays of not more than 500 nm have been intercepted.

The above opaque substrate includes, for example, a substrate in which adye, a pigment, a filler or the like is added to or dispersed in a resinsuch as a polyvinyl resin, a polycarbonate resin, a polyacrylate resin,an amorphous polyolefin resin or the like, an optical disc base board, asubstrate in which an inorganic sputter film, particularly ametal-sputter film is formed on the surface of said resin, and further asubstrate in which a radiation-curable protective film is formed on saidsputter film, particularly the metal-sputter film, and the like. Theshape of the substrate is not limited, and may be, for example,plate-like or film-like.

An article which has as an adhesive layer a cured product of theultraviolet-curable adhesive composition of this invention includes, forexample, information-recording medium, particularly high densityinformation recording disc, representatives of which are optical discssuch as DVD (digital video (or versatile) disc), MO (optical magneticdisc), PD (phase change optical disc) and the like.

Examples are shown below to explain this invention more specifically. Inthe Examples,.parts are by weight.

EXAMPLES 1 TO 5 (THIS INVENTION) AND EXAMPLES 6 TO 9 (COMPARISONS)

Among the various components shown in the following Table 1, the resincomponent was stirred and dissolved at 60° C. in one hour. Thereafter,the polymerization initiator and the like were added to prepareultraviolet curable adhesives for Examples 1 to 5 (this invention) andExamples 6 to 9 (comparisons).

Using each of the compositions obtained, 2 sheets of opaque substrates(aluminum-deposited polycarbonate plates which show a transmittance of0.05% at all wavelengths in a region of not less than 280 nm but notmore than 380 nm) were bonded so that the coating film thickness becameabout 10 μm, and the composition was cured by means of a curingapparatus having a high pressure mercury lamp (80 W/cm). Thereafter, thebonded substrates were peeled and the surface state thereof was observedto evaluate the curability. The results thereof are shown in Table 1.

Using each of the compositions obtained, transparent substrates(polycarbonate plates) were bonded so that the coating film thicknessbecame about 10 μm, and subjected to a peeling strength test at 25° C.to measure a peel value. The results thereof are shown in Table 1.

The abbreviation of each of the compositions shown in the table is asfollows:

EPA-1: Bisphenol type epoxy acrylate (manufactured by NIPPON KAYAKU CO.,LTD.)

UX-6101: Polyester type urethane acrylate (manufactured by NIPPON KAYAKUCO., LTD.)

UX-4101: Polyester type urethane acrylate (manufactured by NIPPON KAYAKUCO., LTD.)

M-315: Tris(acryloxyethyl) isocyanurate (manufactured by TOAGOSEICHEMICAL INDUSTRY Co., Ltd.)

MANDA: Neopentylglycol hydropivalate diacrylate (NIPPON KAYAKU CO.,LTD.)

R-604:5-Ethyl-2-(2-hydroxy-1,1-dimethylethyl)-5-(hydroxymethyl)-1,3-dioxanediacrylate (manufactured by NIPPON KAYAKU CO., LTD.)

HIDDA: 1,6-Hexanediol diacrylate (manufactured by NIPPON KAYAKU CO.LTD.)

R-561: Phenyloxyethyl acrylate (manufactured by NIPPON KAYAKU CO., LTD.)

FA-513A: Tricyclodecane acrylate (manufactured by Hitachi Chemical Co.,Ltd.)

TC-101: Tetrahydrofurfuryl acrylate (manufactured by NIPPON KAYAKU CO.,LTD.)

BP-100: Benzophenone (photopolymerization initiator manufactured byNIPPON KAYAKU CO., LTD., absorption wavelength: 360 nm, molar absorptioncoefficient: not more than 50)

DETX: 2,4-Diethylthioxanthone (photopolymerization initiatormanufactured by NIPPON KAYAKU CO., LTD., absorption wavelength: 360 nm,molar absorption coefficient: not less than 3,000)

Irg-184: 1-Hydroxycyclohexyl phenyl ketone (photopolymerizationinitiator manufactured by Ciba Geigy, absorption wavelength: 360 nm,molar absorption coefficient: not more than 50)

Irg-369: 2-Benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone-1(photopolymerization initiator manufactured by Ciba Geigy, absorptionwavelength: 360 nm, molar absorption coefficient: 3,000)

Irg-1800: Mixture of Irg-184 with bis(2, 6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (photopolyjtierizationi initiatormanufactured by Ciba Geigy, absorption wavelength: 360 nm, molarabsorption coefficient: 1,800)

TPO: 2,4,6-Trimethylbenzoyldiphenylphosphine oxide (photopolymerizationinitiator manufactured by BASF, absorption wavelength: 380 nm, molarabsorption coefficient: 580)

DMBI: Isoamyl p-dimethylaminobenzoate (photopolymerization initiatingassistant manufactured by NIPPON KAYAKU CO., LTD.)

TABLE 1 Example Com- This invention Comparison ponent 1 2 3 4 5 6 7 8 9EPA-1 20 — — 20 — 20 20 20 40 UX-6101 — — 40 — — — — — 10 UX-4101 — 30 —20 35 — — 10 — M-315 — — — 20 — — — 25 — MANDA 30 — 30 — — 30 30 — —R-604 — — — — — — — — 20 HDDA — 35 — — 20 — — — — R-561 50 — — — 45 5050 — — FA-513A — — 30 — — — — — 30 TC-101 — 35 — 40 — — — 45 — BP-100 —— — — — —  5 — — DETX  5 —  5 — — — — — — Irg-184 — — — — —  7 —  7  7Irg-369 — — ‘3 —  5 — — — — Irg-1800 —  3 — — — — — — — TPO — — —  7 — —— — — DMBI  3 —  3 — — —  3 — — Curability ∘ ∘ ∘ Δ ∘ x x x x Bonding1320 1000 800 900 700 180 2640 1420 8 strength (Kgf/mm)

In Table 1, the evaluation criterion of curability is as follows:

◯. . . No tack was found.

Δ. . . Tack was slightly found.

x . . . Not cured at all.

As is clear from Table 1, the compositions of Examples 1 to 5 accordingto this invention are good in curability as compared with the adhesivesof Examples 6 to 9 as comparative examples. Particularly from the dataof Examples 1 to 3 and 5, it is seen that when photopolymerizationinitiators whose molar absorption coefficients at a wavelength of300-450 nm is at least 1,000 were used, the curability was good.Accordingly, the ultraviolet-curable adhesive composition of thisinvention is useful as a suitable ultraviolet-curable adhesive forbonding opaque substrates to one another.

INDUSTRIAL APPLICABILITY

The ultraviolet-curable adhesive composition of this invention can bephotocured in bonding opaque substrates which show transmittances of0.01-20% at all wavelengths in a wavelength region of 280-380 nm, to oneanother and is very useful in bonding, for example, a base board foroptical disc in view of adhesiveness of substrate, protection andproductivity.

What is claimed is:
 1. A method for preparing an information recordingmedium comprising coating the ultraviolet-curable adhesive compositionon a substrate which is a base board for an optical disk, said baseboard showing transmittance of 0.01-20% at all wavelengths of energybeams in the region not less than 280 nm but not more than 380 nm, andthereafter firmly contacting the coating surface of this substrate withanother substrate, and then irradiating any of said substrate, saidanother substrate, or both with ultraviolet rays without the use ofheat; said ultraviolet-curable adhesive composition comprising2,4,6trimethylbenzoldiphenylphosphie oxide as a photopolymerizationinitiator and an ultraviolet-curable compound which is a resin composedof 20-95 w/w % of a monomer having at least one (meth)acryloyl group inthe molecule and 5-80 w/w % of an oligomer having at least two(meth)acryloyl groups in the molecule; said another substrate showstransmittance of 0.01-20% at all wavelengths of energy beams in theregion not less than 280 nm but no more than 380 nm.
 2. The method ofclaim 1, wherein the content of the photopolymerization initiator in theultraviolet-curable adhesive composition is 0.01-20% byweight.
 3. Themethod of claim 2, wherein said content is 0.5-20% by weight.
 4. Themethod of claim 1, wherein said monomer having at least one(meth)acrylolyl group in the molecule is selected from the groupconsisting of phenyloxyethyl (meth)acrylate triecyclodecane(meth)acrylate, isobornyl (meth)acrylate, tetrhydrofurfuryl(meth)acrylate, morpholine acrylate, 2-hydroxyethyl (meth)acrylate,2-hydroxy-3-phenoxypropyl, (meth)acrylate, neopentylglycoldi(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentylglycolhydroxypivalate di(meth)acrylate caprolactone-modified neopentylglycolhydroxypivarate di(meth)acrylate, dipentacrythritol penta(meth)acrylate,dipentacythritol hexa(meth)acrylate,tris((meth)-acryloxyethyl)isocyanurate and carpolactone-modifiedtris(meth)acryloxyethyl) isocyanurate and said oligomer having at leasttwo (meth)acryloyl groups in the molecule is epoxy (meth)acrylate orurethane (meth)acrylate.
 5. The method of claim 1, wherein saidinformation recording medium is a digital video disc.